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GaAs epilayer by hot-wall epitaxy
ARTICLE IN PRESS
Journal of Crystal Growth 279 (2005) 70–75 www.elsevier.com/locate/jcrysgro
Growth and structural properties of rocksalt MnSe/GaAs epilayer by hot-wall epitaxy Y.-M. Yua, D.J. Kima, S.H. Eoma, Y.D. Choia,, M.-Y. Yoonb, I.-H. Choic a
Department of Optical and Electronic Physics, Mokwon University, 800 Doan-Dong, Seo-ku, Daejeon 302-729, Republic of Korea b Division of Information Engineering, Joongbu University, Kumsan 312-702, Republic of Korea c Department of Physics, Chung-Ang University, Seoul 540-742, Republic of Korea Received 1 March 2004; accepted 7 February 2005 Available online 7 April 2005 Communicated by K. Nakajima
Abstract a-MnSe epilayers were grown on (1 0 0) GaAs substrates by hot-wall epitaxy and their structural characteristics were studied. X-ray diffraction (XRD) and double crystal rocking curve measurements revealed that the epilayer is a homogeneous layer of MnSe with a rocksalt structure in the (1 0 0) direction. Asymmetric XRD revealed that biaxial tensile strain remained in a 200 nm thick a-MnSe epilayer. r 2005 Elsevier B.V. All rights reserved. PACS: 61.10.i; 75.50.Pp; 73.61.Ga Keywords: A1. Strain; A3. Hot-wall epitaxy; B1. a-MnSe
1. Introduction Recently, manganese oxides and chalcogenides have attracted considerable interest due to the potential to create new classes of spin-dependent electronic devices with these materials [1]. Among the diluted magnetic semiconductors, Mn-based II–VI ternaries are the most extensively studied and understood. Among the Mn chalcogenides, Corresponding author. Tel.: +82 42 829 7552;
fax: +82 42 823 0639. E-mail address: [email protected] (Y.D. Choi).
MnSe is an antiferomagnetic compound with a cubic NaCl type (a phase) structure and a lattice constant of 5.462 A˚ [2]. The energy gap in a-MnSe has been estimated to be near 2.0 eV by optical absorption and photoemission measurements [3,4]. a-MnSe films have been grown by molecular beam epitaxy (MBE) [5], metalorganic chemical vapor deposition (MOCVD) [6], and thermal evaporation [4,7], but no study has been reported on a-MnSe epilayers grown by hot-wall epitaxy (HWE). HWE is designed to maintain nearthermal equilibrium via the fabrication of a hot wall between the source and the substrate. In
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.02.046
ARTICLE IN PRESS Y.-M. Yu et al. / Journal of Crystal Growth 279 (2005) 70–75
previous studies reported in the literature, Mn chalcogenides epitaxial layers have been grown on a GaAs substrate [5,8–10]. The heteroepitaxial structure has a lattice mismatch, a difference in thermal expansion coefficients, and a difference in chemical properties. These may detrimentally affect crystallinity due to crystal defects generated at the interface. In this study, high-quality a-MnSe epilayers have been grown on GaAs (1 0 0) substrates by HWE. The crystal structure and lattice constant have been examined by X-ray diffraction (XRD) and a double crystal rocking curve (DCRC).
2. Experimental procedure MnSe epilayers were grown on semi-insulating GaAs (1 0 0) substrates by HWE. The source materials were 5N polycrystalline ZnSe and Mn powder. The GaAs substrates were ultrasonically cleaned by trichloroethylene, acetone, and methanol, in sequence, for 5 min each. They were then chemically etched at 50–60 1C in a 3H2SO4:H2O2:H2O solution for 1 min and rinsed by flushing deionized water. After being dried with high purity Ar gas, they were placed on a substrate holder in the HWE system. Before the growth process, preheating at 590 1C under approximately 2 107 Torr pressure for 20 min was carried out to remove remaining impurities and oxide layers on the substrate surface. The substrates were subsequently held at a growth temperature of 200–660 1C. For the growth of a-MnSe epilayers, ZnSe and Mn powders were placed in different sections and heated independently. The temperature for ZnSe was set to 730 1C and that for Mn to 720 1C. Under these growth conditions, Zn2+ ions are almost all replaced by Mn2+ ions. The thickness of the epilayers was about 200 nm, which was determined by scanning electron microscopy (SEM) and reflectance using a spectrophotometer. The growth rate was almost constant over a wide growth temperature range. To determine the chemical composition and the depth profile of the a-MnSe epilayers, energy dispersive X-ray spectrometry (EDX) and secondary ion mass spectrometry
71
(SIMS) were carried out, respectively. The crystal structure and lattice constant were determined by XRD and DCRC.
3. Results and discussion MnSe epilayers were grown on semi-insulating GaAs (1 0 0) substrates by HWE and the growth parameters were varied in order to determine the optimum conditions. The surface morphology was observed using a Nomarski interface microscope. With the naked eye, the surfaces of all samples were mirror-like. Fig. 1 shows the surface morphology of MnSe epilayers grown at 200, 320, 540, and 660 1C. The images in Fig. 1 show that the surfaces of all MnSe epilayers are very flat while few defects are observed. Fig. 2 shows a typical XRD spectrum of the aMnSe epilayers. a-MnSe (2 0 0) and (4 0 0) peaks are revealed along with GaAs (2 0 0) and (4 0 0) peaks, and no other peaks are observed in the spectrum. Therefore, MnSe epilayers grown in a wide growth temperature range (200–660 1C) were found to have a rocksalt structure in the (1 0 0) direction. In all layers grown at growth temperatures higher than 280 1C, the Zn atoms of the source materials were not detected with EDX. However, in some samples grown at the low growth
Fig. 1. Interference microscope images of a-MnSe epilayers grown at 200, 320, 540, and 660 1C
ARTICLE IN PRESS Y.-M. Yu et al. / Journal of Crystal Growth 279 (2005) 70–75
SECONDARY ION COUNT
72
107 6
10
104
Ga
Se
As
3
10
102 101
Zn
0
10
XRD intensity (a.u.)
Mn
105
0
200
400 600 Time (sec)
800
MnSe
MnSe
(200)
(400)
GaAs
GaAs
(200)
(400)
indicate that stoichiometric a-MnSe epilayers were grown on the (1 0 0) GaAs substrate. Fig. 3(a) shows the growth temperature dependence of the full-width at half-maximum (FWHM) of the DCRCs. As the growth temperature increases up to 400 1C, the FWHM decreases, while the FWHM is almost constant up to 600 1C. Above the growth temperature of 600 1C, the FWHM decreases rapidly again. This indicates that the crystallinity improves as the growth temperature increases. This result could be interpreted to mean that the Mn and Se atoms occupy the stable lattice sites, because adatoms for aMnSe growth increase mobility on the surface as the growth temperature increases.
20
30
40
50
60
70
80
2 θ (degree)
DCRC FWHM (arcmin)
24
20
16
12
Fig. 2. Typical XRD spectrum of a-MnSe epilayers. The inset shows the depth profile of a-MnSe epilayers grown at 660 1C.
8 300
400 500 600 Substrate temperature (°C)
(b)
700
5.470
α-MnSe 650 arcsec
-0.5 0.0 0.5 1.0 1.5 2.0 ∆θ (degree)
Lattice constant (Å)
GaAs (400)
DCRC intensity (a.u.)
temperature (o280 1C), Zn atoms were detected in only a small quantity (o7%). The growth condition dependence of Zn incorporation is not clear and is under investigation. To investigate the structural properties of the homogeneous layer of MnSe with a rocksalt structure, we focused our attention on samples grown at 280–660 1C. The composition of the a-MnSe epilayers was determined by EDX, which showed that the ratio of Mn:Se was almost one over a wide growth temperature range (280–660 1C). As seen in the inset of Fig. 2, in the depth profile by SIMS for the a-MnSe epilayers, the concentration of manganese and selenium atoms was almost constant to the depth of the heterointerface, and these elements were not found in the GaAs substrate. However, very few zinc, gallium, and arsenic atoms are observed in the a-MnSe epilayer. These results
(a)
5.465
aα-MnSe = 5.462 Å 5.460
5.455
5.450
(c)
300 400 500 600 700 Substrate temperature (°C)
Fig. 3. (a) Growth temperature dependence of the FWHM of the DCRCs, (b) DCRC spectrum for a 200 nm a-MnSe epilayer grown at 660 1C, and (c) Growth temperature dependence of the lattice constant of the a-MnSe epilayers.
ARTICLE IN PRESS Y.-M. Yu et al. / Journal of Crystal Growth 279 (2005) 70–75
MnSe (511)
XRD intensity (a.u.)
where a? is the a-MnSe lattice constant perpendicular to the interface, as the lattice constant for the GaAs substrate, 5.6533 A˚, ys the Bragg angle for the GaAs substrate, and Dy is the angle separation. For a-MnSe/GaAs epilayers, the lattice constant a? was found to be 5.459 A˚, which is smaller than that of bulk a-MnSe, i.e., 5.462 A˚ [2]. This difference can be explained by the strain in the epilayer. In general, the strain remaining in the aMnSe epilayer will be misfit strain and/or thermal strain. The magnitude of the thermal strain cannot be evaluated, because the thermal expansion coefficient of a-MnSe is unknown at present. In addition, the strain relaxation caused by the gliding of the stacking faults and the formation of the misfit dislocations has been reported in the sample grown/annealed at high temperature [12–14]. To investigate the effects of thermal processing for the a-MnSe epilayers, some asgrown samples were annealed at high temperature (660–800 1C) in a nitrogen atmosphere for 300 s in a rapid thermal annealing system. No strain relaxation was found for the samples annealed below 700 1C. However, relaxation was observed for those annealed above 700 1C. This indicates that the growth temperature (280–660 1C) in this study is not sufficiently high for the enhancement of dislocation gliding. Therefore, the effect of the growth temperature on the strain relaxation is ignored in this work. Fig. 3(c) shows the growth temperature dependence of the lattice constant of the a-MnSe epilayers. On the whole, the lattice constant of the a-MnSe epilayers is found to be smaller than that of the bulk material, which
demonstrates that tensile strain remains in the aMnSe epilayer. However, the lattice constant increased slightly to reach that of the bulk material with increased growth temperature, indicating that the thermal strain in the a-MnSe/GaAs epilayer is compressive. Fig. 4(a) shows an X-ray j-scan curve as a function of the azimuthal angle for the a-MnSe/ GaAs epilayer, which can provide some information about the film phase and the in-plane orientation relationship between the film and the
GaAs (511)
-50
0
50
100
150
200
250
300
Azimuthal angle (degree)
(a)
GaAs MnSe
DCRC intensity (a.u.)
Fig. 3(b) shows a typical DCRC spectrum for a 200 nm thick a-MnSe epilayer grown at a growth temperature of 660 1C. A a-MnSe (4 0 0) peak was observed on the right side of a strong GaAs (4 0 0) peak. This peak shows a FWHM of 650 arcsec, which is superior to that of a 500 nm thick a-MnSe grown by MBE [5]. From the angle separation between the a-MnSe peak and the GaAs substrate peak, the lattice constant for the a-MnSe epilayer can be calculated as [11] sin ys a? ¼ a s , (1) sin ðys þ DyÞ
73
159 arcsec (511)-
(511)+
-0.5 (b)
0.0
0.5
1.0
1.5
2.0
2.5
∆θ (degree)
Fig. 4. (a) XRD j-scans as a function of azimuthal sample rotation for the a-MnSe/GaAs {5 1 1} plane. (b) Asymmetric Xray reflection spectra for a-MnSe/GaAs epilayer for the (5 1 1) reflection [(5 1 1)-] and for the (5 1 1) reflection [(5 1 1)+].
ARTICLE IN PRESS 74
Y.-M. Yu et al. / Journal of Crystal Growth 279 (2005) 70–75
substrate. Four {5 1 1} reflection peaks appear at every 90o, and the peaks in GaAs and a-MnSe are also found at the same azimuthal angle, indicating that the a-MnSe layer is in a single phase and that the [0 1 1] a-MnSe axis and [0 1 1] GaAs axis are parallel. Fig. 4(b) shows the DCRC for the (5 1 1) asymmetric diffraction plane in aMnSe/GaAs. The notation (5 1 1)/(5 1 1)+ refers to the case in which the incident angle is smaller/ larger than the diffraction angle and Dolow and Dohigh are the angle separation between the epilayer and the substrate. The difference of Dolow and Dohigh is small, about 159 arcsec, indicating that the interface strain is not fully relaxed in the 200 nm thick a-MnSe epilayer grown on GaAs. The parallel lattice constant aJ of the a-MnSe epilayer grown on GaAs can be determined by [11] ak ¼ as
sin yB sin f , sin ðyB þ DyÞ sin ðf þ DfÞ
(2)
where Dyand Dfare given by Dolow ¼ Dy Df (2.0521.) and Dohigh ¼ Dy þ Df (2.0961). With the Bragg angle yB (45.0721) for the GaAs (5 1 1) plane and the inclination angle, f (15.7931), the in-plane lattice constant aJ was determined to be 5.467 A˚. This indicates that biaxial tensile strain (9 104) remains in the 0.2-mm thick a-MnSe epilayer grown on GaAs. In a-MnSe/GaAs, the lattice mismatch f ¼ ðasubstrate alayer Þ=alayer ¼ 3.4% is similar to that of a cubic-CdS/GaAs (3.1%) [15,16]. Thus, the critical thickness of the a-MnSe epilayer could be considered to resemble that of 12 A˚ for a cubic CdS epilayer [16]. As the a-MnSe epilayer thickness increases beyond the critical thickness, the strain due to the lattice mismatch relaxes and hence the in-plane lattice constant becomes smaller. However, the strain caused by the lattice mismatch is not completely relaxed in a 0.2-mm thick a-MnSe epilayer [17,18]. The biaxial tensile strain remaining in the 0.2mm thick a-MnSe epilayer results from a residual biaxial tensile strain effect due to the lattice mismatch, while a biaxial compressive strain effect arises from the difference in the thermal expansion coefficient of the a-MnSe epilayer and GaAs substrate. This suggests that there is
mainly residual misfit strain due to the lattice mismatch.
4. Conclusions MnSe epilayers were grown on (1 0 0) GaAs substrates by HWE. XRD spectra showed that the epilayers have a homogeneous layer of MnSe with a rocksalt structure in the (1 0 0) direction over a wide growth temperature range (280–660 1C). The composition of the MnSe epilayers was determined by EDX and the ratio of Mn:Se was almost one to the depth of the heterointerface. The lattice constants were found from DCRC measurements and the FWHM of the DCRC generally decreased as the growth temperature increased. Using asymmetric high resolution XRD, the biaxial tensile strain (9 104) was found to remain in a 200 nm thick a-MnSe epilayer.
Acknowledgment This work was supported by a Korea Research Foundation Grant (KRF-2002-070-C00036).
References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molna´r, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [2] Diluted Magnetic Semiconductors, Semiconductors and Semimetals, in: J. K. Furdyna, J. Kossut (Eds.), vol. 25, Academic Press, Boston, 1988. [3] D.L. Decker, R.L. Wild, Phys. Rev. B 4 (1971) 3425. [4] H. Sato, T. Mihara, A. Furuta, Y. Ueda, H. Namatame, M. Taniguchi, J. Electron. Spectrosc. Relat. Phenom. 78 (1996) 87. [5] H. Kim, R. Vogelgesang, A.K. Ramdas, F.C. Peiris, U. Bindley, J.K. Furdyna, Phys. Rev. B 58 (1998) 6700. [6] P. Tomasini, A. Haidoux, J.C. Te´denac, M. Maurin, J. Crystal Growth 193 (1998) 572. [7] V. Thanigaimani, M.A. Angadi, Thin Solid Films 245 (1994) 146. [8] I. Ishibe, Y. Nabetani, T. Kato, T. Matsumoto, J. Crystal Growth 214/215 (2002) 172. [9] Y.-M. Yu, J.G. Park, M.H. Hyun, S. Nam, O. Byungsung, K.-S. Lee, K.-S. An, Y.D. Choi, M.-Y. Yoon, P.Y. Yu, J. Crystal Growth 237/239 (2002) 1589 and reference therein.
ARTICLE IN PRESS Y.-M. Yu et al. / Journal of Crystal Growth 279 (2005) 70–75 [10] T. Suzuki, I. Ishibe, Y. Nabetani, T. Kato, T. Matsumoto, J. Crystal Growth 237/239 (2002) 1374. [11] V. Swaminathan, A.T. Macrander, Materials Aspects of GaAs and InP Based Structures, Prentice-Hall, Englewood Cliffs, 1991. [12] A. Rodriguez, T. Rodriguez, A. Kling, J.C. Soares, M.F. Dasilva, C. Ballesteros, J. Electron. Mater. 28 (1999) 77. [13] X.W. Liu, A.A. Hopgood, B.F. Usher, H. Wang, N. St. J. Braithwaite, J. Appl. Phys. 94 (2003) 7496.
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[14] J. Ohta, H. Fujioka, S. Ito, M. Oshima, Thin Solid Films 435 (2003) 218. [15] Y.M. Yu, K.S. Lee, O. Byungsung, P.Y. Yu, C.S. Kim, Y.D. Choi, H.J. Yun, J. Vac. Sci. Technol. A 22 (2004) 324. [16] D.W. Niles, H. Hochst, Phys. Rev. B 44 (1991) 10965. [17] S. Nam, J. Rhee, Y.M. Yu, C.K. Lee, O. Byungsung, K.S. Lee, Y.D. Choi, Appl. Phys. Lett. 72 (1998) 2304. [18] K. Wolf, S. Jilka, H. Sahin, H. Stanzl, T. Reisinger, A. Naumov, W. Gebhardt, J. Crystal Growth 152 (1995) 34.
Journal of Crystal Growth 279 (2005) 70–75 www.elsevier.com/locate/jcrysgro
Growth and structural properties of rocksalt MnSe/GaAs epilayer by hot-wall epitaxy Y.-M. Yua, D.J. Kima, S.H. Eoma, Y.D. Choia,, M.-Y. Yoonb, I.-H. Choic a
Department of Optical and Electronic Physics, Mokwon University, 800 Doan-Dong, Seo-ku, Daejeon 302-729, Republic of Korea b Division of Information Engineering, Joongbu University, Kumsan 312-702, Republic of Korea c Department of Physics, Chung-Ang University, Seoul 540-742, Republic of Korea Received 1 March 2004; accepted 7 February 2005 Available online 7 April 2005 Communicated by K. Nakajima
Abstract a-MnSe epilayers were grown on (1 0 0) GaAs substrates by hot-wall epitaxy and their structural characteristics were studied. X-ray diffraction (XRD) and double crystal rocking curve measurements revealed that the epilayer is a homogeneous layer of MnSe with a rocksalt structure in the (1 0 0) direction. Asymmetric XRD revealed that biaxial tensile strain remained in a 200 nm thick a-MnSe epilayer. r 2005 Elsevier B.V. All rights reserved. PACS: 61.10.i; 75.50.Pp; 73.61.Ga Keywords: A1. Strain; A3. Hot-wall epitaxy; B1. a-MnSe
1. Introduction Recently, manganese oxides and chalcogenides have attracted considerable interest due to the potential to create new classes of spin-dependent electronic devices with these materials [1]. Among the diluted magnetic semiconductors, Mn-based II–VI ternaries are the most extensively studied and understood. Among the Mn chalcogenides, Corresponding author. Tel.: +82 42 829 7552;
fax: +82 42 823 0639. E-mail address: [email protected] (Y.D. Choi).
MnSe is an antiferomagnetic compound with a cubic NaCl type (a phase) structure and a lattice constant of 5.462 A˚ [2]. The energy gap in a-MnSe has been estimated to be near 2.0 eV by optical absorption and photoemission measurements [3,4]. a-MnSe films have been grown by molecular beam epitaxy (MBE) [5], metalorganic chemical vapor deposition (MOCVD) [6], and thermal evaporation [4,7], but no study has been reported on a-MnSe epilayers grown by hot-wall epitaxy (HWE). HWE is designed to maintain nearthermal equilibrium via the fabrication of a hot wall between the source and the substrate. In
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.02.046
ARTICLE IN PRESS Y.-M. Yu et al. / Journal of Crystal Growth 279 (2005) 70–75
previous studies reported in the literature, Mn chalcogenides epitaxial layers have been grown on a GaAs substrate [5,8–10]. The heteroepitaxial structure has a lattice mismatch, a difference in thermal expansion coefficients, and a difference in chemical properties. These may detrimentally affect crystallinity due to crystal defects generated at the interface. In this study, high-quality a-MnSe epilayers have been grown on GaAs (1 0 0) substrates by HWE. The crystal structure and lattice constant have been examined by X-ray diffraction (XRD) and a double crystal rocking curve (DCRC).
2. Experimental procedure MnSe epilayers were grown on semi-insulating GaAs (1 0 0) substrates by HWE. The source materials were 5N polycrystalline ZnSe and Mn powder. The GaAs substrates were ultrasonically cleaned by trichloroethylene, acetone, and methanol, in sequence, for 5 min each. They were then chemically etched at 50–60 1C in a 3H2SO4:H2O2:H2O solution for 1 min and rinsed by flushing deionized water. After being dried with high purity Ar gas, they were placed on a substrate holder in the HWE system. Before the growth process, preheating at 590 1C under approximately 2 107 Torr pressure for 20 min was carried out to remove remaining impurities and oxide layers on the substrate surface. The substrates were subsequently held at a growth temperature of 200–660 1C. For the growth of a-MnSe epilayers, ZnSe and Mn powders were placed in different sections and heated independently. The temperature for ZnSe was set to 730 1C and that for Mn to 720 1C. Under these growth conditions, Zn2+ ions are almost all replaced by Mn2+ ions. The thickness of the epilayers was about 200 nm, which was determined by scanning electron microscopy (SEM) and reflectance using a spectrophotometer. The growth rate was almost constant over a wide growth temperature range. To determine the chemical composition and the depth profile of the a-MnSe epilayers, energy dispersive X-ray spectrometry (EDX) and secondary ion mass spectrometry
71
(SIMS) were carried out, respectively. The crystal structure and lattice constant were determined by XRD and DCRC.
3. Results and discussion MnSe epilayers were grown on semi-insulating GaAs (1 0 0) substrates by HWE and the growth parameters were varied in order to determine the optimum conditions. The surface morphology was observed using a Nomarski interface microscope. With the naked eye, the surfaces of all samples were mirror-like. Fig. 1 shows the surface morphology of MnSe epilayers grown at 200, 320, 540, and 660 1C. The images in Fig. 1 show that the surfaces of all MnSe epilayers are very flat while few defects are observed. Fig. 2 shows a typical XRD spectrum of the aMnSe epilayers. a-MnSe (2 0 0) and (4 0 0) peaks are revealed along with GaAs (2 0 0) and (4 0 0) peaks, and no other peaks are observed in the spectrum. Therefore, MnSe epilayers grown in a wide growth temperature range (200–660 1C) were found to have a rocksalt structure in the (1 0 0) direction. In all layers grown at growth temperatures higher than 280 1C, the Zn atoms of the source materials were not detected with EDX. However, in some samples grown at the low growth
Fig. 1. Interference microscope images of a-MnSe epilayers grown at 200, 320, 540, and 660 1C
ARTICLE IN PRESS Y.-M. Yu et al. / Journal of Crystal Growth 279 (2005) 70–75
SECONDARY ION COUNT
72
107 6
10
104
Ga
Se
As
3
10
102 101
Zn
0
10
XRD intensity (a.u.)
Mn
105
0
200
400 600 Time (sec)
800
MnSe
MnSe
(200)
(400)
GaAs
GaAs
(200)
(400)
indicate that stoichiometric a-MnSe epilayers were grown on the (1 0 0) GaAs substrate. Fig. 3(a) shows the growth temperature dependence of the full-width at half-maximum (FWHM) of the DCRCs. As the growth temperature increases up to 400 1C, the FWHM decreases, while the FWHM is almost constant up to 600 1C. Above the growth temperature of 600 1C, the FWHM decreases rapidly again. This indicates that the crystallinity improves as the growth temperature increases. This result could be interpreted to mean that the Mn and Se atoms occupy the stable lattice sites, because adatoms for aMnSe growth increase mobility on the surface as the growth temperature increases.
20
30
40
50
60
70
80
2 θ (degree)
DCRC FWHM (arcmin)
24
20
16
12
Fig. 2. Typical XRD spectrum of a-MnSe epilayers. The inset shows the depth profile of a-MnSe epilayers grown at 660 1C.
8 300
400 500 600 Substrate temperature (°C)
(b)
700
5.470
α-MnSe 650 arcsec
-0.5 0.0 0.5 1.0 1.5 2.0 ∆θ (degree)
Lattice constant (Å)
GaAs (400)
DCRC intensity (a.u.)
temperature (o280 1C), Zn atoms were detected in only a small quantity (o7%). The growth condition dependence of Zn incorporation is not clear and is under investigation. To investigate the structural properties of the homogeneous layer of MnSe with a rocksalt structure, we focused our attention on samples grown at 280–660 1C. The composition of the a-MnSe epilayers was determined by EDX, which showed that the ratio of Mn:Se was almost one over a wide growth temperature range (280–660 1C). As seen in the inset of Fig. 2, in the depth profile by SIMS for the a-MnSe epilayers, the concentration of manganese and selenium atoms was almost constant to the depth of the heterointerface, and these elements were not found in the GaAs substrate. However, very few zinc, gallium, and arsenic atoms are observed in the a-MnSe epilayer. These results
(a)
5.465
aα-MnSe = 5.462 Å 5.460
5.455
5.450
(c)
300 400 500 600 700 Substrate temperature (°C)
Fig. 3. (a) Growth temperature dependence of the FWHM of the DCRCs, (b) DCRC spectrum for a 200 nm a-MnSe epilayer grown at 660 1C, and (c) Growth temperature dependence of the lattice constant of the a-MnSe epilayers.
ARTICLE IN PRESS Y.-M. Yu et al. / Journal of Crystal Growth 279 (2005) 70–75
MnSe (511)
XRD intensity (a.u.)
where a? is the a-MnSe lattice constant perpendicular to the interface, as the lattice constant for the GaAs substrate, 5.6533 A˚, ys the Bragg angle for the GaAs substrate, and Dy is the angle separation. For a-MnSe/GaAs epilayers, the lattice constant a? was found to be 5.459 A˚, which is smaller than that of bulk a-MnSe, i.e., 5.462 A˚ [2]. This difference can be explained by the strain in the epilayer. In general, the strain remaining in the aMnSe epilayer will be misfit strain and/or thermal strain. The magnitude of the thermal strain cannot be evaluated, because the thermal expansion coefficient of a-MnSe is unknown at present. In addition, the strain relaxation caused by the gliding of the stacking faults and the formation of the misfit dislocations has been reported in the sample grown/annealed at high temperature [12–14]. To investigate the effects of thermal processing for the a-MnSe epilayers, some asgrown samples were annealed at high temperature (660–800 1C) in a nitrogen atmosphere for 300 s in a rapid thermal annealing system. No strain relaxation was found for the samples annealed below 700 1C. However, relaxation was observed for those annealed above 700 1C. This indicates that the growth temperature (280–660 1C) in this study is not sufficiently high for the enhancement of dislocation gliding. Therefore, the effect of the growth temperature on the strain relaxation is ignored in this work. Fig. 3(c) shows the growth temperature dependence of the lattice constant of the a-MnSe epilayers. On the whole, the lattice constant of the a-MnSe epilayers is found to be smaller than that of the bulk material, which
demonstrates that tensile strain remains in the aMnSe epilayer. However, the lattice constant increased slightly to reach that of the bulk material with increased growth temperature, indicating that the thermal strain in the a-MnSe/GaAs epilayer is compressive. Fig. 4(a) shows an X-ray j-scan curve as a function of the azimuthal angle for the a-MnSe/ GaAs epilayer, which can provide some information about the film phase and the in-plane orientation relationship between the film and the
GaAs (511)
-50
0
50
100
150
200
250
300
Azimuthal angle (degree)
(a)
GaAs MnSe
DCRC intensity (a.u.)
Fig. 3(b) shows a typical DCRC spectrum for a 200 nm thick a-MnSe epilayer grown at a growth temperature of 660 1C. A a-MnSe (4 0 0) peak was observed on the right side of a strong GaAs (4 0 0) peak. This peak shows a FWHM of 650 arcsec, which is superior to that of a 500 nm thick a-MnSe grown by MBE [5]. From the angle separation between the a-MnSe peak and the GaAs substrate peak, the lattice constant for the a-MnSe epilayer can be calculated as [11] sin ys a? ¼ a s , (1) sin ðys þ DyÞ
73
159 arcsec (511)-
(511)+
-0.5 (b)
0.0
0.5
1.0
1.5
2.0
2.5
∆θ (degree)
Fig. 4. (a) XRD j-scans as a function of azimuthal sample rotation for the a-MnSe/GaAs {5 1 1} plane. (b) Asymmetric Xray reflection spectra for a-MnSe/GaAs epilayer for the (5 1 1) reflection [(5 1 1)-] and for the (5 1 1) reflection [(5 1 1)+].
ARTICLE IN PRESS 74
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substrate. Four {5 1 1} reflection peaks appear at every 90o, and the peaks in GaAs and a-MnSe are also found at the same azimuthal angle, indicating that the a-MnSe layer is in a single phase and that the [0 1 1] a-MnSe axis and [0 1 1] GaAs axis are parallel. Fig. 4(b) shows the DCRC for the (5 1 1) asymmetric diffraction plane in aMnSe/GaAs. The notation (5 1 1)/(5 1 1)+ refers to the case in which the incident angle is smaller/ larger than the diffraction angle and Dolow and Dohigh are the angle separation between the epilayer and the substrate. The difference of Dolow and Dohigh is small, about 159 arcsec, indicating that the interface strain is not fully relaxed in the 200 nm thick a-MnSe epilayer grown on GaAs. The parallel lattice constant aJ of the a-MnSe epilayer grown on GaAs can be determined by [11] ak ¼ as
sin yB sin f , sin ðyB þ DyÞ sin ðf þ DfÞ
(2)
where Dyand Dfare given by Dolow ¼ Dy Df (2.0521.) and Dohigh ¼ Dy þ Df (2.0961). With the Bragg angle yB (45.0721) for the GaAs (5 1 1) plane and the inclination angle, f (15.7931), the in-plane lattice constant aJ was determined to be 5.467 A˚. This indicates that biaxial tensile strain (9 104) remains in the 0.2-mm thick a-MnSe epilayer grown on GaAs. In a-MnSe/GaAs, the lattice mismatch f ¼ ðasubstrate alayer Þ=alayer ¼ 3.4% is similar to that of a cubic-CdS/GaAs (3.1%) [15,16]. Thus, the critical thickness of the a-MnSe epilayer could be considered to resemble that of 12 A˚ for a cubic CdS epilayer [16]. As the a-MnSe epilayer thickness increases beyond the critical thickness, the strain due to the lattice mismatch relaxes and hence the in-plane lattice constant becomes smaller. However, the strain caused by the lattice mismatch is not completely relaxed in a 0.2-mm thick a-MnSe epilayer [17,18]. The biaxial tensile strain remaining in the 0.2mm thick a-MnSe epilayer results from a residual biaxial tensile strain effect due to the lattice mismatch, while a biaxial compressive strain effect arises from the difference in the thermal expansion coefficient of the a-MnSe epilayer and GaAs substrate. This suggests that there is
mainly residual misfit strain due to the lattice mismatch.
4. Conclusions MnSe epilayers were grown on (1 0 0) GaAs substrates by HWE. XRD spectra showed that the epilayers have a homogeneous layer of MnSe with a rocksalt structure in the (1 0 0) direction over a wide growth temperature range (280–660 1C). The composition of the MnSe epilayers was determined by EDX and the ratio of Mn:Se was almost one to the depth of the heterointerface. The lattice constants were found from DCRC measurements and the FWHM of the DCRC generally decreased as the growth temperature increased. Using asymmetric high resolution XRD, the biaxial tensile strain (9 104) was found to remain in a 200 nm thick a-MnSe epilayer.
Acknowledgment This work was supported by a Korea Research Foundation Grant (KRF-2002-070-C00036).
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